Add "-notOnce" to the compiler options on the Mac to stop the compiler

assuming that duplicate #includes of the same file are idempotent.  I mean,
it's not even true for the standard headers (think <assert.h>), and
certainly isn't true here.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2400 cda61777-01e9-0310-a592-d414129be87e